U.S. patent application number 09/746525 was filed with the patent office on 2002-06-27 for bulk acoustic resonator perimeter reflection system.
Invention is credited to Bradley, Paul D., Larson, John D. III, Ruby, Richard C..
Application Number | 20020079986 09/746525 |
Document ID | / |
Family ID | 25001210 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020079986 |
Kind Code |
A1 |
Ruby, Richard C. ; et
al. |
June 27, 2002 |
BULK ACOUSTIC RESONATOR PERIMETER REFLECTION SYSTEM
Abstract
A bulk acoustic resonator having a high quality factor is formed
on a substrate having a depression formed in a top surface of the
substrate. The resonator includes a first electrode, a
piezoelectric material and a second electrode. The first electrode
is disposed on the top surface of the substrate and extends beyond
the edges of the depression by a first distance to define a first
region therebetween. The piezoelectric material is disposed on the
top surface of the substrate and over the first electrode, and the
second electrode is disposed on the piezoelectric material. The
second electrode includes a portion that is located above the
depression. The portion of the second electrode that is located
over the depression has at least one edge that is offset from a
corresponding edge of the depression by a second distance to define
a second region therebetween. The first and second regions have
different impedances, as a result of the different materials
located in the two regions. In addition, the first and second
distances are approximately equal to a quarter-wavelength of a
sound wave travelling laterally across the respective region, such
that reflections off of the edges of the regions constructively
interfere to maximize the reflectivity of the resonator.
Inventors: |
Ruby, Richard C.; (Menlo
Park, CA) ; Larson, John D. III; (Palo Alto, CA)
; Bradley, Paul D.; (Mountain View, CA) |
Correspondence
Address: |
AGILENT TECHNOLOGIES
Legal Department, 51U-PD
Intellectual Property Administration
P.O. Box 58043
Santa Clara
CA
95052-8043
US
|
Family ID: |
25001210 |
Appl. No.: |
09/746525 |
Filed: |
December 21, 2000 |
Current U.S.
Class: |
333/187 ;
310/324; 333/191 |
Current CPC
Class: |
H03H 9/173 20130101;
H03H 9/0211 20130101; H03H 9/132 20130101; H03H 9/02118 20130101;
Y10T 29/42 20150115 |
Class at
Publication: |
333/187 ;
333/191; 310/324 |
International
Class: |
H03H 009/58 |
Claims
What is claimed is:
1. An acoustic resonator comprising: a substrate having a
depression formed in a top surface thereof; a first electrode
disposed on the top surface of the substrate and over the
depression, the first electrode extending beyond a plurality of
edges of the depression by a first distance to define a first
region therebetween; a piezoelectric material disposed on the top
surface of the substrate and over the first electrode; and a second
electrode disposed on the piezoelectric material and including a
portion located above the depression, the portion located above the
depression having at least one edge that is offset from a
corresponding edge of the depression by a second distance to define
a second region therebetween, the second region having an impedance
that differs from an impedance of the first region, wherein an
overlap of the first electrode, the piezoelectric material and the
second electrode forms an acoustic cavity of the resonator.
2. The acoustic resonator of claim 1, wherein each of the first and
second regions has a width that is approximately equal to a
quarter-wavelength of a sound wave travelling laterally across the
respective region.
3. The acoustic resonator of claim 2, wherein the width of the
first region is less than the width of the second region.
4. The acoustic resonator of claim 1, further comprising a
structure disposed on the piezoelectric material, the structure
being located a predetermined distance from an edge of the first
electrode corresponding to the at least one edge of the second
electrode that is offset from the corresponding edge of the
depression, wherein a third region extends from that edge of the
first electrode to the structure, the third region having an
impedance that differs from the impedance of the first region, the
predetermined distance being approximately equal to a
quarter-wavelength of a sound wave travelling laterally across the
third region.
5. The acoustic resonator of claim 4, wherein a fourth region is
defined by the structure, the fourth region having an impedance
that differs from the impedance of the third region, the fourth
region having a width approximately equal to a quarter-wavelength
of a sound wave travelling laterally across the fourth region.
6. The acoustic resonator of claim 4, wherein the structure
includes a plurality of segments, each segment being spaced apart
from a corresponding edge of the first electrode by the
predetermined distance.
7. The acoustic resonator of claim 6, wherein the plurality of
segments are connected to each other, the structure extending
around a plurality of edges of the first electrode.
8. The acoustic resonator of claim 1, further comprising at least
one structure disposed on the piezoelectric material above the
depression, the at least one structure being located a
predetermined distance from the at least one edge of the second
electrode that is offset from the corresponding edge of the
depression, wherein a third region extends from that edge of the
second electrode to the structure, the predetermined distance being
approximately equal to a quarter-wavelength of a sound wave
travelling laterally across the third region, and wherein a fourth
region is defined by the structure, the fourth region having an
impedance that differs from the impedance of the third region, the
fourth region having a width approximately equal to a
quarter-wavelength of a sound wave travelling laterally across the
fourth region.
9. The acoustic resonator of claim 8, wherein two structures are
disposed in parallel on the piezoelectric material above the
depression, the second structure being located a second
predetermined distance from the first structure, the second
predetermined distance being approximately equal to a
quarter-wavelength of a sound wave travelling laterally across a
fifth region between the first and second structures, and wherein a
sixth region is defined by the second structure, the sixth region
having a width approximately equal to a quarter-wavelength of a
sound wave travelling laterally across the sixth region.
10. An acoustic resonator comprising: a substrate having a
depression formed in a top surface thereof, the depression having a
plurality of edges; a first electrode disposed on the top surface
of the substrate and over the depression, the first electrode
having a plurality of edges and extending beyond the edges of the
depression by a first distance to define a first region
therebetween; a piezoelectric material formed on the top surface of
the substrate and the first electrode; and a second electrode
disposed on the piezoelectric material, the second electrode
including a portion located above the depression, a plurality of
edges of the portion located above the depression being offset from
corresponding edges of the depression by a second distance to
define a second region therebetween, the second region having a
second impedance that differs from a first impedance of the first
region, wherein an overlap of the first electrode, the
piezoelectric material and the second electrode forms an acoustic
cavity of the resonator, and wherein the first and second regions
form Bragg reflectors that reflect sound waves back to the acoustic
cavity of the resonator.
11. The acoustic resonator of claim 10, wherein each of the first
and second distances is approximately equal to a quarter-wavelength
of a sound wave travelling laterally across the respective
region.
12. The acoustic resonator of claim 10, wherein the second distance
is greater than the first distance.
13. The acoustic resonator of claim 10, wherein the first distance
is approximately 2 microns.
14. The acoustic resonator of claim 10, further comprising a
structure disposed on the piezoelectric material, the structure
being located a predetermined distance from an edge of the first
electrode, wherein a third region extends from that edge of the
first electrode to the structure, and a fourth region is defined by
the structure, the third region having a third impedance that
differs from both the first impedance and a fourth impedance of the
fourth region.
15. The acoustic resonator of claim 14, wherein the structure
includes a plurality of sections, each section being located from a
corresponding edge of the first electrode by the first
distance.
16. The acoustic resonator of claim 14, wherein each of the
predetermined distance and a width of the fourth region is
approximately equal to a quarter-wavelength of a sound wave
travelling laterally across the respective region.
17. A method of making an acoustic resonator, the method
comprising: providing a substrate having a depression formed in a
top surface thereof and a first electrode disposed on the top
surface, the first electrode being located above the depression and
extending beyond a plurality of edges of the depression by a first
distance to define a first region therebetween; depositing a
piezoelectric material on the top surface of the substrate and over
the first electrode; and depositing a second electrode on the
piezoelectric material, the second electrode including a portion
located above the depression, the portion located above the
depression having at least one edge that is offset from a
corresponding edge of the depression by a second distance to define
a second region therebetween, wherein an overlap of the first
electrode, the piezoelectric material and the second electrode
forms an acoustic cavity of the resonator.
18. The method of claim 17, wherein the first region has a first
impedance that differs from a second impedance of the second
region, each of the first and second distances being approximately
equal to a quarter-wavelength of a sound wave travelling laterally
across the respective region to maximize a reflection coefficient
for lateral modes bouncing off of edges of the acoustic cavity.
19. The method of claim 18, further comprising forming a structure
on the piezoelectric material, the structure being located a
predetermined distance from at least one edge of the first
electrode, wherein a third region extends from the at least one
edge of the first electrode to the structure, the predetermined
distance being approximately equal to a quarter-wavelength of a
sound wave travelling laterally across the third region, and
wherein a fourth region is defined by the structure and has a width
approximately equal to a width of a quarter-wavelength of a sound
wave travelling laterally across the fourth region, the third
region having a third impedance that differs from both the first
impedance and a fourth impedance of the fourth region.
20. The method of claim 17, further comprising forming at least one
structure on the piezoelectric material above the depression, the
at least one structure being located a predetermined distance from
the at least one edge of the second electrode that is offset from
the corresponding edge of the depression, wherein a third region
extends from that edge of the second electrode to the structure,
the predetermined distance being approximately equal to a
quarter-wavelength of a sound wave travelling laterally across the
third region, and wherein a fourth region is defined by the
structure, the fourth region having a width approximately equal to
a quarter-wavelength of a sound wave travelling laterally across
the fourth region.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to acoustic resonators, and
more particularly, to acoustic resonators that may be used as
filters for electronic circuits.
BACKGROUND
[0002] The need to reduce the cost and size of electronic equipment
has lead to a continuing need for small filter elements. Consumer
electronics such as cellular telephones and miniature radios place
severe limitations on both the size and cost of the components
contained therein. Many such devices utilize filters that must be
tuned to precise frequencies. Accordingly, there has been a
continuing effort to provide inexpensive, compact filter units.
[0003] One class of filter elements that has the potential for
meeting these needs is constructed from acoustic resonators. These
devices use bulk longitudinal acoustic waves in thin film
piezoelectric material. In one simple configuration, a layer of
piezoelectric material is sandwiched between two metal electrodes.
The sandwich structure is suspended in air by supporting it around
the perimeter. When an electric field is created between the two
electrodes via an impressed voltage, the piezoelectric material
converts some of the electrical energy into mechanical energy in
the form of sound waves. The sound waves can propagate
longitudinally in the same direction as the electric field and
reflect off the electrode/air interface. In addition, the sound
waves also propagate in a direction transverse to the electric
field and reflect off the various discontinuities at the edges of
the electrodes or the structure.
[0004] The device is a mechanical resonator which can be
electronically coupled. Hence, the device can act as a filter. For
a given phase velocity of sound in the material, the mechanical
resonant frequency is that for which the half-wavelength of the
sound wave propagating longitudinally in the device is equal to the
total thickness of the device. Since the velocity of sound is four
orders of magnitude smaller than the velocity of light, the
resulting resonator can be quite compact. Resonators for
applications in the GHz range may be constructed with physical
dimensions less than 100 microns in diameter and a few microns in
thickness.
[0005] Thin film bulk acoustic resonators (FBARs) and stacked thin
film bulk wave acoustic resonators (SBARs) include a thin sputtered
piezoelectric film having a thickness on the order of one to two
microns. Electrodes on top and bottom sandwich the piezoelectric
film to provide an electric field through the piezoelectric
material. The piezoelectric film, in turn, converts a fraction of
the electric field into a mechanical field. An FBAR is a single
layer of piezoelectric material and acts as an absorption filter.
An SBAR is constructed by stacking two or more layers of
piezoelectric material with electrodes between the layers and on
the top and bottom of the stack. SBARs are typically used as
transmission filters.
[0006] To simplify the following discussion, the present invention
will be explained in terms of an FBAR; however, it will be apparent
from the discussion that the teachings of the present invention are
also applicable to SBARs as well. The portion of the piezoelectric
film included between the overlap of electrodes forms an acoustic
cavity. The primary oscillatory mode of this cavity is that in
which sound waves, of the compression, shear, or plate wave type,
propagate in a longitudinal direction perpendicular to the plane of
the electrodes. Unfortunately, there are other oscillatory modes
that can be excited. These so-called "lateral mode" resonances
correspond to sound waves travelling parallel to the plane of the
electrodes and bouncing off of the walls of the acoustic cavity or
the discontinuity at the edge of the electrode layers. Once in
these lateral modes, the mechanical energy is lost as heat. This
loss of energy affects the quality of the FBAR. Reducing the energy
loss from lateral mode resonances will improve the quality factor
(Q) of the FBAR and permit the design of sharper frequency response
filters, duplexers and oscillators with lower phase noise.
[0007] It is an object of the present invention to provide an
improved, high Q bulk acoustic resonator with reduced energy loss
from lateral mode resonances.
SUMMARY
[0008] In accordance with one embodiment of the present invention,
an acoustic resonator includes a substrate, first and second
electrodes, and a piezoelectric material. The substrate has a
depression formed in a top surface thereof. The first electrode,
which is disposed over the depression in the top surface of the
substrate, to provide an electrode/air interface, extends beyond
the edges of the depression by a first distance to define a first
region therebetween. The piezoelectric material is disposed on the
top surface of the substrate and over the first electrode. The
second electrode is disposed on the piezoelectric material and
includes a portion that is located above the depression. The
portion of the second electrode that is located above the
depression has at least one edge that is offset from a
corresponding edge of the depression by a second distance to define
a second region therebetween. An overlap of the first and second
electrodes and the piezoelectric material forms an acoustic cavity
of the resonator. The first and second regions have impedances that
differ from each other, as a result of the difference in materials
in the two regions. In addition, each of the first and second
distances is approximately equal to a quarter-wavelength of a sound
wave travelling laterally across the respective region, such that
reflections off of the edges of the regions constructively
interfere to maximize the reflectivity of the resonator. Thus, the
first and second regions act as Bragg reflectors and reflect sound
waves from lateral mode resonances back to the acoustic cavity of
the resonator, where these sound waves may then be converted to the
desired, primary oscillatory mode.
[0009] The acoustic resonator includes a further perimeter
reflection system to reflect additional sound waves from lateral
mode resonances back to the acoustic cavity of the resonator. The
perimeter reflection system can include structures disposed on the
piezoelectric material around the first electrode or structures
disposed on the piezoelectric material above the depression and
around the second electrode. An example of the former includes a
structure located a predetermined distance from an edge of the
first electrode corresponding to an edge of the second electrode
that is offset from a corresponding edge of the depression. A third
region extends from that edge of the first electrode to the
structure. The third region has an impedance that differs from that
of the second region, and the predetermined distance is
approximately equal to a quarter-wavelength of a sound wave
travelling laterally across the third region. The structure itself
defines a fourth region having an impedance different from that of
the third region, and the width of the fourth region is
approximately equal to a quarter-wavelength of a sound wave
travelling laterally across the fourth region.
[0010] A example of the latter structure is one disposed on the
piezoelectric material above the depression and located a
predetermined distance from an edge of the second electrode that is
offset from a corresponding edge of the depression. A third region
extends from that edge of the second electrode to the structure,
and the predetermined distance is approximately equal to a
quarter-wavelength of a sound wave travelling laterally across the
third region. Similar to the previous example, the structure itself
defines a fourth region having an impedance different from that of
the third region, and the width of the fourth region is
approximately equal to a quarter-wavelength of a sound wave
travelling laterally across the fourth region.
[0011] In accordance with another embodiment of the invention, a
method of making an acoustic resonator is described. The method
includes providing a substrate having a depression formed in a top
surface thereof and a first electrode disposed on the top surface.
The first electrode is located above the depression and extends
beyond the edges of the depression by a first distance to define a
first region therebetween. The method further includes depositing a
piezoelectric material on the top surface of the substrate over the
first electrode and depositing a second electrode on the
piezoelectric material. An overlap of the first and second
electrodes and the piezoelectric material forms an acoustic cavity
of the resonator. The second electrode includes a portion located
above the depression that has at least one edge that is offset from
a corresponding edge of the depression by a second distance to
define a second region therebetween. The second region has an
impedance that differs from that of the first region. In addition,
each of the first and second distances is approximately equal to a
quarter-wavelength of a sound wave travelling laterally across the
respective region, and the first and second regions form Bragg
reflectors. Additional structures, such as those described above,
can also be added to reflect more sound waves from lateral mode
resonances back to the acoustic cavity of the resonator.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The present invention may be better understood, and its
numerous object, features, and advantages made apparent to those
skilled in the art by referencing the accompanying drawings,
wherein like reference numerals are used for like parts of the
various drawings.
[0013] FIG. 1 is a cross-sectional view of an FBAR resonator.
[0014] FIG. 2 is a cross-sectional view of an SBAR resonator.
[0015] FIG. 3 is a top view of an FBAR in accordance with one
embodiment of the present invention.
[0016] FIG. 4 is a cross-sectional view taken generally along the
line 4-4 of FIG. 3.
[0017] FIG. 5 is a cross-sectional view taken generally along the
line 5-5 of FIG. 3.
[0018] FIG. 6 is a top view of an FBAR in accordance with another
embodiment of the present invention.
[0019] FIG. 7 is a cross-sectional view taken generally along the
line 7-7 of FIG. 6.
DETAILED DESCRIPTION
[0020] The present invention may be more easily understood with
reference to FIGS. 1 and 2, which are cross-sectional views of an
FBAR and an SBAR, respectively. Referring to FIG. 1, an FBAR 10
formed on a substrate 12 includes bottom and top electrodes 14 and
18, respectively, which sandwich a sheet of piezoelectric material
16. The piezoelectric material 16 is suspended over a depression 20
to provide an electrode/air interface on the bottom of the FBAR.
The depression 20 is typically created by etching away a portion of
substrate 12. The preferred piezoelectric material is aluminum
nitride, AlN, however other piezoelectric materials may also be
used. The electrodes 14 and 18 are preferably made of molybdenum;
however, embodiments employing other materials may also be
constructed. A coordinate system 22 is oriented such that the
z-axis corresponds to longitudinally directed waves of any mode
type, while the x-axis and y-axis refer to transversely directed
waves of the compression, shear or plate-mode type.
[0021] These devices are designed to use bulk compression or shear
acoustic waves propagating in a direction parallel to the z-axis in
the thin film piezoelectric material as the desired resonator mode.
When an electric field is created between the two electrodes via an
impressed voltage, the piezoelectric material converts some of the
electrical energy into mechanical energy in the form of sound
waves. The sound waves propagate in the same direction as the
electric field shown at 24 and reflect off of the electrode/air
interface.
[0022] At the mechanical resonance, the device appears to be an
electronic resonator; hence the device can act as a notch filter.
The mechanical resonant frequency is the frequency for which the
half-wavelength of the sound waves travelling in the device is
equal to the total thickness of the device for a given composite
phase velocity of sound in the material. Since the velocity of
sound is four orders of magnitude smaller than the velocity of
light, the resulting resonator can be quite compact. Resonators for
applications in the GHz range may be constructed with physical
dimensions of the order of 100 microns in length and a few microns
in thickness.
[0023] FIG. 2 is a cross-sectional view of an SBAR 30. An SBAR
provides electrical functions analogous to those of a band-pass
filter. The SBAR 30 is basically two FBAR filters that are
mechanically coupled. The depression under the bottom layer of
piezoelectric material has been omitted from this drawing. A signal
across electrodes 32 and 34 at the resonance frequency of the
piezoelectric layer 36 will transmit acoustic energy to the
piezoelectric layer 38. The mechanical oscillations in the
piezoelectric layer 38 are converted to an electrical signal across
electrodes 34 and 40 by the piezoelectric material.
[0024] Referring again to FIG. 1, when a potential is applied
across the electrodes in the z-direction to generate the desired
wave type, a transversely directed mechanical strain may be
generated, which can excite sound waves travelling laterally within
the piezoelectric layer. These sound waves are predominately
reflected by the abrupt change in density at the edges of the
electrodes shown at 24, but may also be reflected by the edges of
the depression or the edges of the piezoelectric sheet. A portion
of the sound waves is not reflected by the edges but instead passes
across the edge and is lost as heat. It is this loss of energy due
to these lateral modes that reduces the quality factor (Q) of the
FBAR.
[0025] FIGS. 3-5 illustrate an embodiment of an FBAR 50 in
accordance with one embodiment of the present invention. FIG. 3 is
a top view of the FBAR, and FIGS. 4 and 5 are cross-sectional views
taken generally along lines 4-4 and 5-5, respectively. The FBAR 50
is formed on a top surface 52 (FIGS. 4 and 5) of a substrate 54 and
is situated above a depression 56 in the substrate 54 to provide an
electrode/air interface for the bottom of the FBAR. The FBAR 50
includes a top electrode layer 58, a layer of piezoelectric
material 60 and a bottom electrode layer 62. In FIG. 3, the bottom
electrode layer 62 and the depression 56 are indicated by dashed
lines, as both are located underneath the layer of piezoelectric
material 60. The top electrode layer 58 includes a portion 64 that
extends over the depression 56 in the top surface 52 of the
substrate 54. An overlap of the portion 64 of the top electrode
layer 58, the piezoelectric material 60 and the bottom electrode
layer 62 defines the acoustic cavity of the FBAR.
[0026] The FBAR 50 of the present invention is an improved FBAR
with a higher Q than conventional FBARs. This is due to a perimeter
reflection system surrounding the FBAR that reduces the loss of
energy from the FBAR. The perimeter reflection system includes a
plurality of regions that act as Bragg reflectors to recycle energy
from lateral mode resonances and return that energy to the acoustic
cavity so that it may be converted back to the primary oscillatory
mode of the FBAR.
[0027] The perimeter reflection system includes a first region 66
that extends from edges 68a, 68b and 68c of the portion 64 of the
top electrode layer 58 to corresponding edges 70a, 70b and 70c of
the depression 56 and a second region 72 that extends from edges
70a, 70b and 70c of the depression 56 to corresponding edges 72a,
72b and 72c of the bottom electrode layer 62. As best illustrated
in FIGS. 4 and 5, the first region 66 includes the layer of
piezoelectric material 60 and a portion of the bottom electrode 62
that is located over the depression 56. The second region 72
includes the layer of piezoelectric material 60, the bottom
electrode 62, and the substrate 54. As a result of the different
materials which comprise the first and second regions, the first
region has an impedance that differs from that of the second
region. The impedance of the first region also differs from that of
the acoustic cavity of the FBAR, as the first region does not
include the top electrode 58.
[0028] The first region 66 has a width or distance d.sub.1 that is
equal to the distance from an edge 68a, 68b or 68c of the top
electrode 58 to the corresponding edge 70a, 70b or 70c of the
depression 56. Similarly, the second region 72 has a width or
distance d.sub.2 which is the distance between an edge 70a, 70b or
70c of the depression 56 and the corresponding edge 74a , 74b or
74c of the bottom electrode 62. Each distance d.sub.1 and d.sub.2
is approximately equal to a quarter-wavelength of a sound wave
travelling laterally across the respective region. In the preferred
embodiment of the invention the distance d, is greater than the
distance d.sub.2, and the distance d.sub.2 is approximately 2
microns. Because of the mismatch of impedances between the acoustic
cavity of the FBAR and the first region and between the first and
second regions, a percentage of the sound waves from lateral mode
resonances are reflected back to the acoustic cavity of the FBAR
50. Thus, the arrangement of the first and second regions 66 and 72
enables the FBAR 50 to suppress lateral mode resonances.
[0029] The perimeter reflection system further includes a structure
76 disposed proximate the FBAR 50. In the preferred embodiment of
the invention, the structure 76 includes a plurality of segments
78a, 78b and 78c and surrounds all but one side of the FBAR 50.
However, one of ordinary skill in the art will appreciate that the
structure 76 may include fewer segments. In addition, the segments
need not be connected to one another. Each segment 76a, 76b and 76c
is offset from the corresponding edge 74a, 74b and 74c of the first
electrode by a distance d.sub.3. Thus, a third region 80 having a
width equal to the distance d.sub.3 is formed between the edges
74a, 74b and 74c of the bottom electrode and the structure 76. The
third region 80 has an impedance that differs from that of the
second region 72. This difference in impedance is due to the
absence of the bottom electrode layer 62 in the third region 80.
Thus, the third region 80 includes only the layer of piezoelectric
material 60 and the substrate 54. The distance d.sub.3 of the third
region 80 is selected such that it is approximately equal to a
quarter-wavelength of a sound wave travelling laterally across the
third region 80, so that reflections off of the edges of the third
region will constructively interfere.
[0030] The structure 76 is preferably made of the same material as
the top and bottom electrodes 58 and 62, respectively, and may be
deposited on the layer of piezoelectric material 60 at the same
time as the top electrode 58. The structure 76 has a width d.sub.4.
A fourth region 82 having the width d4 includes the structure 76,
the piezoelectric layer 60 and the substrate 54. The fourth region
82 has an impedance that differs from the impedance of the third
region 80. The width d.sub.4 of the fourth region 82 is selected
such that it is approximately equal to a quarter-wavelength of a
sound wave travelling laterally across the fourth region 82, so
that reflections off of the edges of the fourth region 82 will
constructively interfere.
[0031] Thus, in the perimeter reflection system of the present
invention the distances d.sub.1, d.sub.2, d.sub.3 and d.sub.4 of
the respective first region 66, second region 72, third region 80
and fourth region 82, are carefully selected such that for lateral
mode resonances of the FBAR 50, sound waves reflecting off of the
edges of each region constructively interfere to maximize the
reflection coefficient of the FBAR 50. As a result, the regions act
as Bragg reflectors, and energy from lateral mode resonances, which
would have been lost, is reflected back to the acoustic cavity of
the FBAR 50. Once reflected back to the acoustic cavity, this
energy may be converted to the desired primary oscillatory mode of
the FBAR, which is in a longitudinal direction perpendicular to the
plane of the electrodes.
[0032] Although it is not illustrated in the drawings, one of
ordinary skill in the art will appreciate that additional
structures similar to structure 76 may be added at intervals around
structure 76 to increase the percentage of energy from lateral mode
resonances that is brought back to the acoustic cavity of the FBAR.
The more Bragg reflector regions there are in the perimeter
reflection system, the closer the perimeter reflection system will
be to a perfect mirror that reflects all of the energy back to the
acoustic cavity of the FBAR.
[0033] The FBAR 50 with the perimeter reflection system may be
constructed, as follows, on the substrate, which can be, for
example, a conventional silicon wafer, on which FBARs are
manufactured simultaneously. For purposes of simplicity, however,
the discussion will be limited to the manufacture of a single FBAR.
The depression 56 is first etched into the top surface 52 of the
substrate 54. Next a thin layer of thermal oxide is grown on the
top surface 52 to prevent phosphorous from the
phosphor-silica-glass (PSG), which will be used in a subsequent
step, from diffusing into the substrate 54. Such a diffusion would
convert the silicon forming substrate into a conductor, which would
interfere with the electrical operation of the final device.
[0034] A sacrificial PSG layer (not shown) is then deposited on the
top surface 52 at a temperature of approximately 450.degree. C.,
using silicane and P.sub.2O.sub.5 to form a soft, glass-like
material which is approximately 8% phosphorous. The PSG layer is
then polished using a slurry to remove the portions of the PSG
outside of the depression 56 and to leave a "mirror-like" finish on
top of the PSG portion in the depression 56. The substrate is then
cleaned.
[0035] After cleaning an electrode layer is deposited and
selectively etched to form the bottom electrode 62 of the FBAR 50.
Various materials, such as molybdenum, aluminum, tungsten, gold,
platinum and titanium, may be used for electrodes. Molybdenum has a
low thermoelastic loss, making it advantageous for use in
resonators.
[0036] After the bottom electrode 62 has been deposited, the layer
of piezoelectric material 60 is deposited. In one embodiment the
piezoelectric layer is a sputter-deposited layer of AIN having a
thickness in the range of approximately 0.1 micron and 10 microns.
The top electrode layer 58 and the structure 76, which are formed
of the same material as the bottom electrode 62, are then deposited
on the layer of piezoelectric material 60 and selectively
etched.
[0037] Next, the bottom side of the substrate may be thinned using
a lapping, plasma etch, or chemical mechanical polishing (CMP)
process to remove material from the underside of the substrate,
thereby improving thermal properties of and reducing an
electromagnetic influence in a resulting filter. The sacrificial
PSG layer and thermal oxide layer may be removed from the
depression 56 at any time after the bottom electrode 62 has been
deposited. In the preferred embodiment of the invention, the PSG
layer and thermal oxide layer are removed after the substrate has
been thinned. Vias (not shown) are formed in the substrate to
expose the PSG layer, and the PSG layer and thermal oxide layer are
removed by etching in a dilute H.sub.2O:HF solution. The resulting
FBAR is illustrated in FIGS. 3-5.
[0038] FIGS. 6 and 7 illustrate another embodiment of an FBAR 50'
with a perimeter reflection system in accordance with the present
invention. FBAR 50' has a perimeter reflection system that includes
a first and second multi-element reflectors. The first
multi-element reflector has elements 84a, 84b and 84c, and the
second multi-element reflector has elements 86a, 86b and 86c.
Additional multi-element reflectors may be included, but are not
illustrated for reasons of simplicity.
[0039] As best illustrated in FIG. 7, unlike structure 76 in FIGS.
3-5, which is not located above the depression 56 formed in the
substrate 54, the first and second multi-element reflectors are
located above the depression 56 to provide a more effective
structure. Reflector elements 84a, 84b, 84c, 86a, 86b and 86c are
higher acoustic impedance elements. Each of reflector elements 84a,
84b and 84c is located a distance d.sub.1' away from a
corresponding edge of top electrode 58 and has a width d.sub.2'.
Similarly, each of reflector elements 86a, 86b and 86c is located a
distance d.sub.3' away from respective reflector elements 84a, 84b
and 84c and has a width d.sub.4'. Reflector elements 86a, 86b and
86c are located a distance d.sub.5' from the respective edges 70a,
70b and 70c of the depression 56 formed in the substrate 54, and
edges 70a, 70b and 70c of the depression 56 are located a distance
d.sub.6' from respective edges 74a, 74b and 74c of the bottom
electrode 62. The distances d.sub.1', d.sub.3', d.sub.5' and
d.sub.6' widths d.sub.2' and d .sub.4' are precisely chosen such
that each is approximately equal to a quarter-wavelength of a sound
wave travelling laterally across the particular region, in order to
reflect lateral modes back to the FBAR 50' in phase.
[0040] Additional multi-element reflectors or structures similar
structure 76 of FIGS. 3-5 may be provided outside of the depression
56 around the first and second multi-element reflectors. The FBAR
50' with multi-element reflectors may be made in a manner similar
to that described above with respect to the FBAR 50 of FIGS.
3-5.
[0041] As described above, the present invention provides improved
FBARs that result in less energy from lateral mode resonances. The
perimeter reflection system of the FBAR and surrounding structure
forms a series of Bragg reflectors that redirect energy from
lateral mode resonances back to the acoustic cavity of the FBAR,
where that energy may be converted to the desired longitudinal
mode. The present invention provides an improved FBAR with a high Q
that will permit the design of sharper frequency response filters,
duplexers and oscillators with lower phase noise.
[0042] While particular embodiments of the present invention have
been shown and described, it will be obvious to those of ordinary
skill in the art that changes and modifications may be made without
departing from this invention in its broader aspects. For example,
even though the acoustic cavity of the FBARs shown has a
rectangular shape, the acoustic cavity may take on any polygonal
shape. In particular, it may be beneficial for the acoustic cavity
to have an irregular polygonal shape (e.g., and irregular
quadrilateral) in order to reduce the absorption anomalies caused
by the lateral modes. The perimeter reflection system of the
present invention may be designed for an FBAR with a
non-rectangular acoustic cavity. Thus, the appended claims are to
encompass within their scope all such changes and modifications as
fall within the true spirit and scope of this invention.
* * * * *